This invention relates to a method of beneficiating kish graphite to high purity levels in an economic manner without undue destruction of the coarse graphite flakes.
Graphite, particularly flake graphite, finds use in many applications, including refractories, as a coating material in foundries, in brake linings, batteries, carbon brushes, pencils, propellants, as a precursor for expandable graphite, as a lubricant, and in other areas.
In refractories, flake graphite imparts high thermal conductivity and improved slag resistance. At operating temperatures, residual impurities are detrimental to the high temperature stability of the flakes. Because of this there is a tendency in refractory applications to use flake graphite of higher purity. For example, U.S. Pat. No. 4,912,068 describes the significant increase in hot strength made possible by using 99% pure flake graphite in magnesia-carbon brick. Flake graphite sold to certain battery markets requires a purity level of at least 99.9% carbon. Flake graphite sold to the expandable graphite market requires a purity of at least 96%. Large, high purity flakes command a higher price and are becoming increasingly desired. All of these tendencies are driving graphite producers to find new methods of producing high purity flake graphite. Further, it is important to note that the United States is totally dependent on foreign sources for this material. Therefore, the desire exists to find a suitable domestic source of this important raw material.
Flake graphite occurs naturally as flakes disseminated in metamorphosed siliceous or lime-rich sediments such as limestone, gneiss, and schist. In unweathered deposits it is difficult to liberate and separate the flakes from the ore without destroying their size. A number of processes are utilized to upgrade flake graphite to high purity levels, that is above 96% carbon, without significantly decreasing the size of the flakes.
In addition to naturally occurring graphite, there is a type of graphite referred to as "kish" graphite.
"Kish" is a general term for waste that forms during the iron production process and during the initial steelmaking process. It contains graphite in a flake form, iron, lime-rich slag, and other materials. Efforts have been made to recover flake graphite from kish and it is this graphite that has been now termed "kish graphite".
To date, efforts to improve the purity of kish graphite and still retain its coarse size have not been commercially satisfactory. They have generally followed the accepted procedures utilized for upgrading graphite-containing ores which require the use of multi-step chemical leaching. Such a process is described in the Bureau Of Mines preliminary Report No. 415 of Jan. 1993 and the extension of this work by the joint effort of Inland Steel Industries and Asbury Graphite Mills, Inc. which was presented at the 86th Annual Meeting of the Air & Waste Management Association on Jun. 13-18, 1993. These techniques are able to upgrade the purity of the graphite to 95-96% carbon, but at significant cost due to the numerous leaching steps required. Further upgrading of the kish graphite to 98 or 99% purity has also been reported in the Bureau of Mines report noted above, but, again, with multiple steps using strong acids bearing the attendant cost of having all of the safety features involved for handling of the acids, recovering, and disposing of the same.
In short, it has not been possible to economically beneficiate kish graphitic to 99% graphitic carbon content.